This invention relates to a cellular communication network and method which has particular utility in underground mines, but also application in industry and localised open environs.
The underground mining industry has long recognised the importance of radio communications for enhancing productivity and safety. Experiments on radio transmission in underground mines date back to as early as the 1920s. Most of these early experiments were aimed at through-the-rock radio communication between the surface and underground mine personnel. However, past experience has indicated that through-the-rock radio transmission is too sensitive to rock composition for it to be considered reliable.
Starting at the end of the 1960s through to the 1970s, extensive experimental and theoretical investigations on electromagnetic propagation in underground tunnels were carried out in Europe and USA. These studies led to a good understanding of the complex mechanisms involved in radio propagation in underground mine, road and railway tunnels as well as the developments of leaky feeders and mode converters for improving radio propagation along tunnels. Radio systems employing leaky coaxial cables are still widely used in underground mines. However, these systems are usually restricted to supporting only one or two voice channels and remain expensive for continually extending the communication distance required in conjunction with the ongoing extension of the mine in a normal mining area application.
In recent years, the sophistication of mechanical equipment used in modern underground mines has grown rapidly with advanced technology but the progress in underground communications remains almost stagnant. In addition to voice communication, modern underground mines require data communication for remote monitoring and control of machinery. In these cases, data communication is usually achieved with a separate wired system operating independently from the radio system for voice communication.
In most underground mines, voice communication is achieved via either line telephone or radio systems based on low-frequency inductive loop or high-frequency leaky feeder techniques. In an attempt to improve communication quality, in recent times there has been a focus on VHF and UHF leaky coaxial cable systems. A drawback with these systems, however, is that the higher cable attenuations encountered at these high frequencies mean that signal amplifications by intermediate repeaters are needed at approximately every 500 m to 1500 m. It follows that a good understanding of the complex operating mechanisms of leaky coaxial cable is essential for achieving the required system performance. Furthermore, these leaky systems require rewiring for each advance of the active mining area.
For remote monitoring and control purposes, it is common to use a separate data transmission system based on metallic cables, such as twisted pairs and coaxial cable, or more recently optical fibre cable. Manually operated data loggers are sometimes employed for data acquisition in smaller mines.
These existing systems remain very restrictive in terms of communication capacity, mobility, voice quality and flexibility in system reconfiguration and extension. Despite the high investment in installing, maintaining and operating an array of incompatible systems, the performance achieved has been less than satisfactory. Therefore it is believed that a significant cost advantage can be achieved by adopting an integrated system capable of handling both voice and data.
Having regard to surface communication networks, in recent times the cellular concept of radio coverage has become well established in the wide area cellular telephone field. Moreover, applications involving smaller cell sizes for servicing local areas in offices, factories and residential areas are beginning to be introduced. An example is the advanced Digital European Cordless Telecommunications system (DECT). In this system, fixed radio base stations are suitably located to provide radio coverage for particular zones of the service area. Usually, each base station is linked by a cable to a central control hub in a star configuration. For most of these surface applications, the links between the base stations and the central hub are short. As such, a star network is appropriate. However, a very different situation is encountered in underground mining, and hence conventional surface cellular communication networks cannot be employed.
Moreover, in an underground mining situation, the structural layout of an underground mine is typified by a series of tunnels and crossings which severely restrict radio transmission. Consequently, severe radio propagation losses are encountered along tunnels and around corners and crossings.
Accordingly, it is an object of the present invention to provide for a cellular communication network and method within an area, and parts and accessories thereof capable of handling both voice and data communications to provide improved operational efficiency and flexibility as well as reducing the cost otherwise needed for maintaining separate communication systems within said area.
It is a preferred object of the invention to provide reliable communications for multiple channels of two-way voice, monitoring data, control signals and compressed digital video signals within the network.
It is a further preferred object of the invention to provide a communication network which is capable of automatic tracking and locating of objects such as personnel and equipment within said area.
In accordance with a first aspect of the present invention, there is provided a communication network for an area including:
a plurality of cascaded base stations serially interconnected in a ring structure to form a network backbone;
one of said base stations at one end of said network backbone comprising a network controller for controlling the network;
a plurality of portables adapted for communication with any of said base stations by a common air interface;
a network protocol for communicating between said network controller and said base stations or between said base stations along said network backbone; and
a common air interface protocol for communicating between a said portable and a said base station;
wherein the other base stations function as slaves to said network controller, and
wherein each slave base station defines a discrete cell within which communication between a said portable disposed within said cell and said base station thereof, is able to be performed over said common air interface.
Such a network has particular utility in an underground mine, whereby the network not only enhances voice communication between mining personnel, but allows the use of distributed monitoring and control of both personnel and mining equipment. Furthermore, the communication network can be continually extended with the further progression of the mine, to provide for mine wide communications and thus provide an immediate warning to workers of dangerous situations and locations, thus improving the overall safety level of underground mines.
In accordance with a second aspect of the present invention, there is provided a base station for a communication network of the type defined in the preceding aspect of the invention, including one or more backbone controller means for communicating with the network backbone, a common air interface controller means for communicating with any of the portables within the cell of the base station, and a backplane means for controlledly interconnecting said backbone controller means, said common air interface controller means and said network backbone.
In accordance with a third aspect of the present invention, there is provided a network controller for a communication network of the type defined in the first aspect of the invention, including a communications microcontroller provided with a serial communication channel for operating the network controller and communicating with the network backbone, a microcomputer bus communications interface for connecting said communications microcontroller to a microcomputer, and a backbone communications interface connecting said communications microcontroller to the network backbone.
In accordance with a fourth aspect of the present invention, there is provided a portable for a communication network of the type defined in the first aspect of the invention, including: a communications microcontroller provided with a plurality of serial communication channels (SCC) for operating the portable and generating timing signals for enabling communication with a base station over the common air interface; a burst mode RF transceiver for receiving data from and transmitting data to a base station over the common air interface under the control of said communications microcontroller; an adaptive differential pulse coded modulation (ADPCM) means to encode analog signals input to the ADPCM means for transmitting to a portable, into a digitised bit stream for said communications microcontroller to transmit over the common air interface, and in reverse to decode digitised bit streams to be output by the ADPCM means as analog signals received from the common air interface and which are input to said communications microcontroller; peripheral devices; auxiliary control circuits; and an I2C serial bus and bus controller for allowing communication and control between said communications microcontroller and said peripheral devices and said auxiliary control circuits.
In accordance with a fifth aspect of the invention, there is provided a method for communicating along a communication network as defined in the first aspect of the invention including:
initialising the network controller and all of the base stations in the network;
assigning prescribed operating frequencies for each of the slave base stations in sequence; and
linking each of the slave base stations into the network backbone to commence interbase station and intrabase station communications.
In accordance with a sixth aspect of the invention, there is provided a method for establishing a common air interface (CAI) between a portable and a communication network as defined in the first aspect of the invention, said method including:
scanning through all of the carrier frequencies of the base stations connected into the network;
selecting the carrier frequency of a base station having the maximum received field strength;
listening to messages transmitted by said base station at the selected carrier frequency and identifying a free time slot on the airlink with said base station;
transmitting a log-on message to said base station after a prescribed time period from the end of said free time slot; returning an acknowledgment message from said base station to said portable after a further prescribed time period corresponding to the position of said free time slot; and
communicating the identity of said portable from said base station to said network controller via the network backbone.
In accordance with a seventh aspect of the invention, there is provided a method for communicating between a portable and another party in a communication network as defined in the first aspect of the invention, said method including:
establishing a common air interface (CAI) between said portable and a base station a t the carrier frequency of said base station;
searching for a free time slot of sufficient capacity on the airlink with said base station for the particular communication;
upon finding said free time slot, sending a communication request message and identity codes of both said portable and said other party from said portable to said base station during said free time slot;
relaying the request to said network controller from said base station via the network backbone
directing the request from said network controller to the base station of said other party;
selecting a free time slot of sufficient capacity on the airlink of said other party base station for said communication;
notifying said other party of the request, from the base station thereof, if sufficient capacity is available on the airlink of said base station thereof, or if there is not sufficient capacity terminating the communication; waiting for an acknowledgment from said other party after said notifying;
sending a message of said acknowledgment from said base station of said other party to said base station of said portable via said network controller; and allowing communications to proceed between said portable and said other party.
In accordance with an eighth aspect of the invention, there is provided a method for communicating between a portable and a communication network as defined in the first aspect of the invention, said method including:
establishing a common air interface (CAI) between a portable and a base station at the carrier frequency of said base station;
continuously monitoring the field strength of signals received by said portable from said base station with which said CAI is established;
searching for a stronger field strength of other base stations at different carrier frequencies with which a CAI is not established, upon said field strength of the established CAI falling below a prescribed threshold;
switching said portable to the carrier frequency of a new base station having a CAI with a stronger field strength above said threshold after said searching; establishing a CAI between said portable and said new base station; and
disabling the CAI with the old base station having a field strength below said threshold.